Everybody seems to know about silica gel and cosmoline - both of which are pretty good, but so far I see no mention of oxygen absorbing packets. They're a one-time use only product but they work far better than anything else because they eliminate the cause of oxidation - the oxygen! The only thing you have to worry about is letting them come in contact with the steel of the gun. If that happens they'll cause pitting at the site of contact. Since oxygen comprises about 16% of our atmosphere (at sea level) they also cause a slight vacuum in the sealed container. They're not expensive, though you probably won't find them at either your favorite gun store or Wal Mart, instead having to order them from a supplier. Google is your friend.